Capacitive transducers convert variations in capacitance generated between a diaphragm and a fixed electrode to electrical signals. An example of the condenser type electroacoustic transducer is an electret condenser microphone unit. A condenser microphone unit experiences a significant decrease in sensitivity at harmonic frequencies of the resonance frequencies of the diaphragm vibrated in a low order vibration mode. Thus, a condenser microphone unit should be designed to have a frequency band of sound collection below the resonance frequency of the diaphragm.
High-resolution audio sources, which are also referred to as “high-resolution audio,” have recently been in demand. The maximum frequency of audio sources recorded on typical CDs is 22.05 kHz. In contrast, the maximum frequency of audio sources in high-resolution is 40 kHz or higher. Thus, a microphone supporting high-resolution audio should be responsive to a frequency of 40 kHz or higher. Unfortunately, conventional condenser type electroacoustic transducers cannot readily achieve a resonance frequency of 40 kHz or higher because of the tension characteristics of the diaphragms. If the dimensions of the diaphragm are reduced to shift the resonance frequency, the driving force is reduced. Thus, conventional condenser type electroacoustic transducers cannot achieve a sufficiently high S/N ratio because an increase in resonance frequency to support high resolution audio sources weakens the output signal.
There is a demand for a condenser microphone unit that can reduce a decrease in sensitivity at harmonic frequencies of the resonance frequency of the diaphragm and stably collect sound in high-frequency bands.
For example, an electret capacitive transducer is disclosed that has an electret layer having a surface charge distribution in which the central area has a smaller charge than that of the peripheral area (for example, refer to Japanese Examined Patent Publication No. 62-39880). Another speaker is also disclosed that includes a circular diaphragm attached to a circular sub-diaphragm at a junction such that the outer circumferential section of the circular sub-diaphragm is connected to the circular diaphragm at a region including the anti-node of a secondary resonance mode of the circular diaphragm (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-281669).
A piezoelectric speaker is disclosed that includes a diaphragm including multiple curved surfaces disposed parallel to each other and including a piezoelectric element attached to the junction of the diaphragm (for example, refer to Japanese Unexamined Patent Application Publication No. 9-215093).